As others have noted, there really
is little real incentive. In classical antiquity, rule of thumb is that the wealth is in the east. The only ones I can see even attempting to get more land in the west than OTL would be the Maurya dynasty. I understand
@fjihr's reasoning as to Persia, but I also note that prior to Chandragupta, there wasn't ever a unified all-Indian empire. So short of inventing an ATL unifier who forges an empire in India, Chandragupta and his heirs are the best bet. The reason I'm hammering on unifying all India is that, as previously noted, spending energy conquering Tamilakam, Sri Lanka etc. is a much better investment. So anyone sane will do that first. The Mauryas are the best shot at succeeding in it, and
then you can (somewhat) plausibly have them start thinking of pushing further west. Though to be honest, if I found myself in that position, I'd try to annex the area of modern-day Myanmar, Malaysia and (parts of) Indonesia. Still a better investment than pushing west.
But okay, here's a POD: Seleukos gets killed when Chandragupta pushes west as per OTL. There is no Seleukid empire. Easern Persia falls into disarray as vassal kings and satraps set up their own fiefs, while the Successors wage war in the west (as per OTL). Cleverly using this situation, Chandragupta establishes his overlordship over the Baktrians (which is a smart step, because that region holds the key position for northern trade with China: by conquering it, Chandragupta is making himself the unavoidable middleman for future trade between west and far east).
After that, he gets going (as per OTL) with expanding and solidifying his hold on India. This might actually go better if he holds Baktria, which is actually a profitable possession due to its position on a trade route, and will increase his funds. Perhaps he can even get some pretty well-trained Greco-Baktrian troops for his armies. Might be useful. In any case, let's suppose that due to this, he can annex Kalinga, so Ashoka doesn't have to. If we also assume that any future Maurya heir (alt-Ashoka, if you will) is still a highly competent ruler, we may see interesting things yet. For starters, such a ruler could then subdue the Tamil dynasties in the south (not having to spend time on Kalinga, after all). After that, and with increased holdings, more wealth, and less enemies nearby compared to OTL, the Mauyas can plausibly start looking for... new areas to conquer.
I still think that going for the general area of modern-day Myanmar, Malaysia and (parts of) Indonesia would make the most sense. After all, in this scenario, they already monopolise the overland trade with the east. If they also control everything down to the straits at the location of OTL Singapore, they also monopolise any sea-trade with the far east. At that point, I suspect the Mauryas rapidly become the most wealthy power on earth. After all, in OTL, the trade of the classical age was essentially a lot of gold going east, and little gold flowing west. The east had it all, the west had basically nothing the east wanted. But that's comparing the states and empires around the Med on the one hand, the the 'east' (Persia, India and China) on the other hand. When it came to trade between India and China, it went both ways. These two had things to offer each other. So if the Mauryas control all trade between the far east and eveything west of India, they are botyh exporters (to both the west and China) as well as the middlemen in the east-west trade who can get a profit on all trade flowing through their empire. Basically, they'll be very rich without ever suffering the trade balance issues that the western powers faced when trading with the east.
After that, you can easily see a scenario where India basically just becomes the world's foremost superpower and essentially colonises the shit out of everything, much like Europe did in OTL. Including everything to their west. That meets the challenge, so... there you go. Of course, they might
still be interested in colonising in a (south-)eastern direction, first. Given the natural wealth available, I know that
I would! So maybe we need to give them a better reason to go west? Say all of this happens, and in the meantime, the ATL Wars of the Successors have wound down, and have resulted in some other ruling dynasty than the Seleukids governing an empire that's essentially Persia. It's not entirely unrealistic that this empire might want to "take back" Baktria etc. They will probably also be very annoyed at the economic supremacy and general trade monopolising of the Maurya dynasty. So, conceivably... they might launch a war on Maurya India. Which Maurya India wins
spectacularly, after which point they basically just annex the enemy's empire... ending up with a dominion that extends to the Fertile Crescent.
I still think it's not a very plausible scenario-- but it's not outright impossible.